1. Field of the Invention
The present invention relates to a solid-state imaging device having a large number of pixels arranged in a row direction and in a column direction, which is perpendicular to the row direction.
2. Description of the Related Art
In a single-chip color solid-state imaging device typified by a CCD or CMOS image sensor, three or four kinds of color filters are arranged like a mosaic on an array of light receiving portions adapted to perform photoelectric conversion. Consequently, a color signal corresponding to each of the color filters is outputted from a corresponding one of the light receiving portions. A color image is generated by processing such color signals.
However, in the related color solid-state imaging device in which color filters are arranged like a mosaic, in a case where the filters are primary color filters, the color filters absorb nearly (⅔) of incident light. Thus, the related color solid-state imaging device has problems in that light utilization efficiency is poor, and that sensitivity is low. Also, because each of the light receiving portions can obtain only a single-color signal, the related color solid-state imaging device has problems that a resolution is low, and that false colors are highly visible.
Thus, to overcome such problems, an imaging device configured to stack three layers of photoelectric conversion film on a semiconductor substrate, on which a signal read circuit is formed, has been studied and developed (see, for example, JP-T-2002-502120 and JP-A-2002-83946). The imaging device has a light receiving portion structure in which, for example, photoelectric conversion film layers respectively adapted to generate signal charges (electrons or positive holes) in response to blue (B) light, green (G) light, and red (R) light are sequentially stacked from a light incidence plane. Additionally, each of the light receiving portions is provided with a signal read circuit enabled to independently read a signal charge optically generated in each photoelectric conversion film.
In the case of the imaging device of such a structure, almost all of incident light is photoelectrically converted and is read out. Thus, the use efficiency of visible light is nearly 100%. Additionally, color signals of three colors R, G, B are obtained at each of the light receiving portions. Consequently, this imaging device can generate a favorable image with high sensitivity and high resolution (thus, false colors are unnoticeable in this image).
An imaging device described in JP-T-2002-513145 is provided with a triple well (photodiode), which is adapted to detect an optical signal, in a silicon substrate. Thus, the imaging device obtains signals that respectively having different spectral sensitivities depending on depths in the silicon substrate and also has peaks at the wavelengths of B (blue), G (green) and R (red) from a surface thereof. This imaging device utilizes the fact that the penetration depth of incident light in the silicon substrate depends on the wavelength thereof. This imaging device can obtain an image with high sensitivity and high resolution (thus, false colors are unnoticeable), similarly to the imaging devices described in JP-T-2002-502120 and JP-A-2002-83946.
However, the imaging devices described in JP-T-2002-502120 and JP-A-2002-83946 are required to stack three layers of photoelectric conversion film on a semiconductor substrate in sequence and to form longitudinal wires that connect signal charges, which are generated in the photoelectric conversion layers respectively corresponding to R, G, B, to the signal read circuits formed on the semiconductor substrate. Thus, these imaging devices have problems that these imaging devices are difficult to manufacture, and the manufacturing yields of these devices are low, and that the manufacturing costs of these devices are high.
Meanwhile, the imaging apparatus described in JP-T-2002-513145 is configured so that blue light is detected by the photodiode at the shallowest part, that red light is detected by the photodiode provided at the deepest part, and that green light is detected by the photodiode at an intermediate part. However, for example, optical charges are generated by the photodiode provided at the shallowest part from green light or red light. Thus, this imaging device has a problem that the separation of the spectral sensitivity characteristics of R signal, G signal and B signal is insufficient and that thus, the color reproducibility is low. Also, it is necessary for obtaining net R-, G- and B-signals to perform addition/subtraction operations on output signals of each of the photodiodes. Thus, this imaging device has another problem that the addition/subtraction operations deteriorate the S/N ratios of image signals.
To solve the problems of the imaging devices described in JP-T-2002-502120, JP-A-2002-83946 and JP-T-2002-513145, an imaging device described in JP-A-2003-332551 has been proposed. This imaging device is a hybrid type of the imaging devices described in JP-T-2002-502120, JP-A-2002-83946 and JP-T-2002-513145. The imaging device described in JP-A-2003-332551 is configured so that B-light and R-light are detected by photodiodes which are provided in a silicon substrate, and that G-light is detected by a photoelectric conversion element provided on the silicon substrate. The photoelectric conversion element provided on the silicon substrate includes a first electrode film stacked on the silicon substrate, a photoelectric conversion film which is stacked on the first electrode film and is made of an organic material, and a second electrode film stacked on the photoelectric conversion film. Signal charges generated in the photoelectric conversion film by applying a voltage to each of the first electrode film and the second electrode film are transferred to the first electrode film and the second electrode film. A signal corresponding to the signal charge transferred to one of the electrode films is read by a signal read circuit, such as a CCD or CMOS circuit, provided in the silicon substrate. In the present specification, the term “photoelectric conversion film” is defined as a film adapted to absorb light having been incident thereon and also having a specific wavelength, and to generate electrons and positive holes according to an amount of the absorbed light.
With this configuration, it is sufficient to provided only one layer of the photoelectric conversion film in this imaging device, so that the manufacturing process is simplified, and that both of increase in the cost and decrease in the yield can be prevented. Green light is absorbed by the photoelectric conversion film. Thus, this imaging device has advantages in that separation of the spectral sensitivity characteristics of the photodiodes respectively corresponding to blue light and red light can be improved, that the color reproducibility of the device can favorably be enhanced, and that the S/N ratio is improved.
In the silicon substrate provided with the two photodiodes adapted to detect B-light and R-light, and with the signal read circuits, the mobility of electros is about 3 times that of positive holes. Therefore, generally, n-channel MOS transistors are used as transistors constituting the signal read circuits. Accordingly, electrons are utilized as electric charges taken from the photoelectric conversion film provided in the silicon substrate.
However, photoelectric conversion films made of an organic semiconductor are often used. It is known that according to the general properties of the organic semiconductor, the mobility of positive holes is larger than the mobility of electrons. Thus, in a case where electrons, whose mobility is smaller than that of positive holes, are utilized as electric charges taken from the photoelectric film used to detect G-light, the probability of occurrence of annihilation of electrons during transfer is high. Also, the probability of trapping electrons at a trap level is high. Consequently, the sensitivity for G-light may be lowered.
Meanwhile, in a case where light is incident on the second electrode film from above, the photoelectric conversion film made of an organic semiconductor relatively largely generates electrons in the vicinity thereof in response to light having a wavelength, at which an optical absorption coefficient is large. Therefore, in a case where electrons are captured by the first electrode film, and where light has a wavelength at which the optical absorption coefficient is large, the electrons are transferred a long distance. Consequently, the sensitivity for light having a wavelength, at which the optical absorption coefficient is large, is very largely reduced. Meanwhile, the photoelectric conversion film made of an organic semiconductor generates electrons substantially uniformly in the photoelectric conversion film in response to light having a wavelength at which the optical absorption coefficient is relatively small. Therefore, reduction in the sensitivity is not large, as compared with the reduction in the sensitivity in the case of using light having a wavelength, at which the optical absorption coefficient is large. Thus, in a case where G-light is detected by the photoelectric conversion film in the hybrid type imaging device, the spectral sensitivity characteristic for G-light is adapted so that peak sensitivity is reduced, and that the distribution of the sensitivity is shaped like a gently sloped mountain. Therefore, the color separation characteristic for G-light is degraded. Consequently, the color reproducibility of a color reproduction image is reduced. Even in a case where R-light and B-light are detected by the photoelectric conversion film, the color reproducibility is reduced. However, especially, the reduction in the color reproducibility is significant in the case of detecting G-light.